Telomeres are nucleoprotein complexes at the ends of linear eukaryotic chromosomes, which are distinguished from DNA double-strand breaks (DSBs). To maintain genomic integrity, all organisms respond to DSBs by promptly launching the DNA-damage response. This response involves the recruitment of DNA repair factors to sites of DNA damage and the activation of signal transduction pathways, often termed DNA-damage checkpoint pathways. Telomeres escape from checkpoint activation and DNA repair. Paradoxically, checkpoint and repair proteins are essential for telomere maintenance. The long-term goal of this project is to uncover the regulatory mechanism of how telomeres collaborate with DSB response machinery. DSBs are repaired by homologous recombination (HR) or non-homologous end joining (NHEJ). The Mre11-Rad50-Nbs1 (Xrs2 in budding yeast) complex recognizes DSBs and promotes HR and NHEJ repair, whereas the Ku complex binds to DSB ends and stimulates the NHEJ pathway. The ATM kinase, which is highly conserved from yeast to human, plays a key role in activating the checkpoint pathway in response to DSB induction. ATM kinase is encoded by TEL1 in budding yeast. ATM/Tel1 localizes to DSBs by interacting Mre11-Rad50-Nbs1/Xrs2 (MRN/MRX) complex. How telomere proteins modulate the ATM/Tel1 checkpoint and Ku-mediated NHEJ pathway remains to be determined. Double-stranded portions of telomeres are covered with telomere-specific DNA binding proteins including TRF1 and TRF2. TRF2 interacts with and recruits RAP1 to telomeres. Human TRF2-RAP1 complex inhibits the ATM checkpoint and NHEJ pathways, although the molecular detail is not fully understood. In budding yeast, Rap1 (the RAP1 homolog) binds directly to double-stranded telomere sequences and plays a key role in telomere homeostasis. Budding yeast Rap1 recruits several telomere-binding proteins including Rif1 to telomeres. In this proposal, we plan to uncover the molecular detail of how Rif1 controls Tel1 accumulation and DSB repair at DNA ends (Aim 1), and define the mechanism of how Rap1 inhibits MRX accumulation at DNA ends (Aim 2). We will also determine how Ku localizes to telomeres while telomeres inhibit the NHEJ pathway (Aim 3). Given the evolutionary conservation of DNA repair and telomere-binding proteins, our study using budding yeast will provide invaluable information to understand how human telomeres modulate DSB responses. Since improper telomere maintenance is implicated in carcinogenesis and cell senescence, our study will contribute to the development of better cancer treatment and the prevention of premature aging. PUBLIC HEALTH RELEVANCE: It is crucial to understand how telomeres modulate the DNA double-stranded break (DSB) response, because improper telomere maintenance leads to cancer development and cell senescence. Since DNA repair and telomere-binding proteins are conserved from yeast to human, our study using budding yeast will show the right direction to understand the picture of how human telomere-binding proteins regulate DSB repair and checkpoint response.